1. Field of the Invention
The present invention relates to a mirror signal generation method for generating a mirror signal from a disc read signal and the like, a mirror signal generation circuit, and an optical disc device using the same.
2. Description of the Related Art
Optical discs of various standards such as CD-R/RW, DVD-R/RW, Blue ray, and the like are being widely used. In such optical disc device, tracking error signal and mirror signal are known as signals indicating the moving distance and the moving direction when performing the pickup movement such as track jump and seek operation. The mirror signal is a signal obtained by comparating and digitizing the signal indicating on-track/off-track at a predetermined slice level. An RF ripple signal (hereinafter appropriately expressed as RFRP signal) and the like having an amplitude magnitude of the disc read signal (hereinafter appropriately expressed as RF signal in the present specification) can be used as the signal indicating the on-track/off-track state.
A patent document 1 shows a general mirror signal detection circuit. In the configuration of FIG. 11 of the patent document 1, two peak bottom hold circuits having different droop rates are arranged, where an envelope difference signal of the RF signal is retrieved at the first stage and peak bottom hold is performed at the second stage. In this method, the level of the peak hold and the bottom hold are added and divided by 2 to generate a slice level of the envelope difference signal.
Various methods such as method of changing the addition ratio of the peak bottom hold level of the second stage, devising a constant to be multiplied after the addition etc. are being reviewed regarding the slice level generation method, but in either method, the slice level is generated with the peak bottom hold level of the second stage as a reference.
[Patent document 1] Japanese Patent Application (Laid Open) No. 9-128761
If noise is mixed in the RF signal or the amplitude level is suddenly changed, the slice level does not stabilize and a correct mirror signal cannot be obtained with the above method.